Hashicorp Configuration Language

External

• https://www.terraform.io/docs/configuration/index.html

Internal

• Terraform Concepts

Comments

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Block

Terraform Block

terraform {
  required_version = ">= 0.12"
}

Resource Block

Declares a resource.

Data Block

Declares a data source.

data "data-source-name" "local-name" {
  ...
}

data "aws_ami" "example" {

  most_recent = true

  owners = ["self", "0000000"]

  filter {
    name   = "name"
    values = ["consul-ubuntu-*"]
  }

  tags = {
    Name   = "app-server"
    Tested = "true"
  }
}

Module Block

Represents a module call.

References to Named Values or Interpolation Expression

https://www.terraform.io/docs/configuration/expressions.html#references-to-named-values

Resource elements can be used by other resources via interpolation expressions:

resource "aws_subnet" "something" {

  vpc_id = "${aws_vpc.main.id}"
  ...
}

Input Variable

https://www.terraform.io/docs/configuration/variables.html

variable "region" {
  description = "This is the region"
  type = string
  default = "us-east-1"
}

Each variable can have a default value specified with "default". "default" may be "null". If there is no "default" declaration, Terraform will signal an error when terraform apply is executed, and it will initiate an interactive CLI that will inquire for the value of the missing variable:

var.environment_name
  The name of the environment

  Enter a value:

variable "masters" {
  description = "The number of master nodes"
  type = number
  default = 1
}

Variables can be used via interpolation expression, prefixed with “var.”

To use the variable:

...
Name = "${var.playground_name}-playground-vpc"
...

Variables can be assigned in multiples way, and this is the descending order of precedence:

• command-line flags (-var = ‘region=something’)
• from a file (.tfvars, -var-file=…) terraform.tfvars or *.auto.tfvars in the current directory are automatically loaded. Multiple -var-file can be used.
• From environment variables: TF_VAR_name. This can only be used for string variables.
• UI Input
• Variable defaults (“default” keyword).

Lists

https://learn.hashicorp.com/terraform/getting-started/variables#lists

variable "security_groups" {
  description = "The list of security groups"
  type        = list
  default     = ["sg-a41f9d51704199e97"]
}

Maps

https://learn.hashicorp.com/terraform/getting-started/variables#maps

Output Variable

https://www.terraform.io/docs/configuration/outputs.html

https://learn.hashicorp.com/terraform/getting-started/outputs

Output variables are a way to organize data to be easily queried and shown back to the Terraform user. Terraform usually stores hundred or thousands of attribute values, but only a few of those are important. Output variables have several uses: a child module can use outputs to expose a subset of its resource attributes to a parent module, a root module ca use outputs to print certain values in the CLI output after running terraform apply and when using remote state, module outputs can be accessed by other configurations via a terraform_remote_state data source.

"Output value" and "output" are semantically equivalent.

An output is declared as:

output "bastion_public_ip" {
  value = aws_instance.bastion.public_ip
  description = "The public IP address of the bastion"
}

Also see:

terraform output

Functions

https://www.terraform.io/docs/configuration/functions.html

https://www.terraform.io/docs/configuration/expressions.html#function-calls

Function Categories

String

substr

https://www.terraform.io/docs/configuration/functions/substr.html

...
cidr_block = "${substr("${var.my_cidr_block}", 0, 5)}.1.0/24"
...

IP Network

cidrsubnet

https://www.terraform.io/docs/configuration/functions/cidrsubnet.html

The following turns "10.10.0.0/16" into its first /24 subnet "10.10.1.0/24"

...
cidr_block = "${cidrsubnet("${var.vpc_cidr_block}", 8, 1)}"
...